5-year-old agression

My almost 5 year old has developed a new behavior at school. In the past 3 weeks he has become extremely agressive with his classmates. He has kicked, hit, pushed, thrown mud at, and bitten different classmates. Some of these incidents are provoked while others are completely unprovoked. He has also developed disruptive behavior in class and screaming outbursts with little provication. I've taken him to the doctor to rule out any physical malady, but I'm stumped to figure out why he's become agressive and disruptive at school.

I am a single mom who works full time. DS's father is in the picture generally, but has remarried and is expecting another son within the next 4 months. There is a step-brother that lives with DS's father.

Has anyone else experienced anything like this? Is this normal for a 5 year old? I feel like I'm walking on egg shells around him to make sure he has enough sleep, food, and affection (the affection part is easy) so he doesn't get out of control with his anger. Would a growth spurt spawn this type of behavior? He is going through a fairly large growth spurt at the moment.